In most eukaryotic cells mRNA levels vary widely depending upon their turnover and differential rates of transcription of their genes. Considerable information is available on the role of abundant mRNAs. Comparatively little is known about rare mRNAs in eukaryotic cells. To understand the importance of the rare mRNAs in cell function, we have isolated a cDNA clone for rare mRNA from a cDNA library generated from lactating rat mammary gland. This cDNA clone codes for a protein of Mr24,000 in an in vitro system. The mRNA corresponding for this cDNA clone is present both in mammary gland and in liver and is of the same size in both organs. However, there is an altered expression of this mRNA in some mammary tumors as well as in some hepatomas. Expression of this mRNA in the mammary gland is under the control of prolactin, where as the expression in liver is under the control of androgens, glucocorticoids and thyroid hormones.